CHEMISTRY OF DIGESTION AND NUTRITION. 227 



found than could be held in combination by the bases present in the secretion. 

 Evidently, some of the chlorine must have been present in combination with 

 hydrogen as hydrochloric acid. Confirmatory evidence of one kind or another 

 has since been obtained. Thus it has been shown that a number of color 

 tests for free mineral acids react with the gastric juice : methyl- violet solutions 

 are turned blue, congo-red solutions and test-paper are changed from red to 

 blue, 00 tropseolin from a yellowish to a pink-red, and so on. A number of 

 additional tests of the same general character will be found described in the 

 laboratory handbooks of physiology. 1 It must be added, however, that lactic acid 

 undoubtedly occurs, or may occur, in the stomach during digestion. Ite pres- 

 ence is usually explained as being due to the fermentation of the carbohydrates, 

 and it is therefore more constantly present in the stomach of the herbivora. 

 The amount of free acid varies according to the duration of digestion ; that is, 

 the secretion does not possess its full acidity in the beginning, owing probably 

 to the fact (Heidenhain) that in the first periods of digestion, while the secre- 

 tion is still scanty in amount, a portion of its acid is neutralized by the 

 swallowed saliva and the alkaline secretion of the pyloric end of the stomach 

 (see the section on Secretion). Estimates of the maximum acidity in the 

 human stomach are usually given as between 0.2 and 0.3 per cent. The 

 acidity of the dog's gastric juice is greater 0.3 to 0.58 per cent. 



Origin of the HC1. The gastric juice is the only secretion of the body con- 

 taining a free acid. The fact that the acid is a mineral acid makes this circum- 

 stance more remarkable, although other instances of a similar kind are known; 

 for example, Dolium galea, a mollusc, secretes a salivary juice containing free 

 H 2 SO 4 and free HC1. When and how the HC1 is formed in the stomach is 

 still a subject of investigation. Histologically, attempts have been made to show 

 that it is produced in the border cells of the peptic glands in the fundic end 

 of the stomach (see Secretion). It cannot be said, however, that the evidence 

 for this theory is at all convincing; it can be accepted only provisionally. 

 Ingenious efforts have been made to determine the place of production of the 

 acid by micro-chemical methods. Substances which give color reactions with 

 acids have been injected into the blood, and sections of the mucous membrane 

 of the stomach have then been made to determine microscopically the part of 

 the gastric glands in which the acid is produced ; but beyond proving that the 

 acid is formed in the mucous membrane these experiments have given negative 

 results, the color reaction for acid occurring throughout the thickness of the 

 membrane. 2 The chemistry of the production of free HC1 also remains unde- 

 termined. No free acid occurs in the blood or the lymph, and it follows, there- 

 fore, that it is manufactured in the secreting cells. It is quite evident, too, 

 that the source of the acid is the neutral chlorides. of the blood; these are in 

 some way decomposed, the chlorine uniting with hydrogen to form HC1 which 

 is turned out upon the free surface of the stomach, while the base remains 



1 Stirling : Outlines of Practical Physiology. 



2 Frankel : Pfliigefs Archivfilr die gesammte Physiologic, 1891, vol. 48, p. 63. 





